Abstract
Early workers in the field of tissue culture attempted to establish various cell types from embryos and other sources which could be propagated indefinitely as a stock cell strain. These attempts stemmed from observations on cultured chick heart fibroblasts by Carrel, who reported to have maintained these cells for 28 months in serial culture. His hypothesis was that primary cell explants could be cultured in vitro for indefinite periods of time (Carrel, 1914). For many years, the work and reports of Carrel went unchallenged, but the results of more recent studies suggested that the behavior of primary cells in culture pointed in another direction.